1. Field of the Invention
The present invention relates to a surface acoustic wave device which operates by generating a surface acoustic wave containing as a major component an SH (shear horizontal) type wave such as a BGS (Bleustein-Gulyaev-Shimizu) wave, a Love wave, or other such wave, and, more particularly, to an edge-reflection type surface acoustic wave device.
2. Description of the Related Art
Surface acoustic wave devices are used in a wide variety of devices such as resonators, bandpass filters, and other such electronic components. With ordinary surface acoustic wave devices, it is necessary to form reflectors on both sides of an interdigital transducer (IDT), which necessarily increases the size of surface acoustic wave devices.
In order to cope with this problem, there has been proposed an edge-reflection type resonator which uses an SH type surface acoustic wave. In the edge-reflection type resonator, an IDT is disposed on a piezoelectric substrate. The SH type surface acoustic wave excited by the IDT is reflected between the edges of the piezoelectric substrate which are located on opposite sides of the IDT. Thus, the two edges opposite to each other are used to reflect the surface acoustic waves instead of using the reflectors to reflect the waves. Accordingly, it is unnecessary to provide the reflectors.
An example of the above-described edge-reflection type resonator is disclosed in Japanese Unexamined Patent Publication No. 60-41809. In this publication, there is disclosed an edge-reflection type resonator which utilizes an SH type surface acoustic wave. FIG. 8 is a schematic plan view of an edge-reflection type resonator which is described as a conventional example in the above prior art publication.
An edge-reflection type resonator 51 includes a piezoelectric substrate 52 having a rectangular plate shape. On the upper surface 52a thereof, a pair of interdigital electrodes (comb-shaped electrodes) 53 and 54 are provided so as to define one IDT. The interdigital electrodes 53 and 54 have a plurality of electrode fingers, respectively, which are interdigitated with each other. In the edge-reflection type resonator 51, each of the electrode fingers of the interdigital electrodes 53 and 54 is composed of a split electrode pair, that is, two split electrodes, except for the electrode fingers 53a and 53c which are located at the outermost sides in the surface acoustic wave propagation direction. For example, the electrode finger 53b of the interdigital electrode 53 is composed of split electrodes 53b1 and 53b2. Each of the electrode fingers 54a-54c of the interdigital electrode 54 is composed of a pair of two split electrodes, that is, 54a1 and 54a2, 54b1 and 54b2, and 54c1 and 54c2, respectively.
The surface acoustic wave propagation direction is perpendicular to the lengthwise direction of the electrode fingers 53a-53c and 54a-54c. The excited surface acoustic wave is reflected between the two edges 52b and 52c disposed opposite to each other, and thereby, resonant characteristics can be obtained.
As described above, by use of the electrode fingers, each composed of two split electrodes, that is, a pair of the split electrodes, the desired band characteristics, which can not be obtained if the ordinary single type electrode fingers are used, can be attained.
In the conventional edge-reflection type resonator 51 as shown in FIG. 8, the ratio of each electrode finger to the gap between the electrode fingers is 1:1, though it is not clearly stated in the above prior art, and ordinarily, the width of the split electrode is .lambda./8 in which .lambda. is the wave length of an excited surface acoustic wave. In the prior art, the widths of the electrode fingers 53a and 53c at the outermost sides in the surface acoustic wave propagation direction are equal to the width of the respective split electrode fingers which constitute the remaining electrode fingers. Further, the resonator 51 is arranged so that the reflection end surfaces are located at the centers of a set of the two electrode fingers which are positioned at the outermost sides, respectively, or the distance between one of the centers and the corresponding reflection end-surfaces are an integral multiple of .lambda./2.
However, in the case of the edge-reflection type resonator constructed according to the above-described prior art, there is a problem that the ratio of the anti-resonant resistance Ra to the resonant resistance Rr, that is, the ratio of "the top" to "the bottom" is not sufficient. In addition, there is a problem that unnecessary ripples are produced in the frequency characteristics, and good band characteristics can not be achieved.